S.S.
I'm sorry but I'm an adult and I have a problem using other washrooms. Perhaps bribery would help. Seriously, or if she's nervous that someone is listening or requiring her to perform perhaps you could explain to her that she is very loved and that it is safe and okay. I know I know not much help, but I am still waiting for someone to explain that to me. In the meantime bring a change of clothes... and give her a hug, she'll come around.